1. Field of the Invention
The present invention relates to a toner for electrophotography and a developer for electrophotography which can be utilized in an electrophotography device utilizing an electrophotography process such as a copying machine, a printer, or a facsimile, as well as to an image forming method.
2. Description of the Related Art
Regarding electrophotography methods, many methods are already known (see, for example, Japanese Patent Application Publication (JP-B) No. 42-23910). Generally, a fixed image is formed after a plurality of steps of electrically forming a latent image on a surface of a photoreceptor (latent image retaining body) utilizing a photoconductive substance by a variety of means, developing the formed latent image using a toner for electrophotography (hereinafter, also simply referred to as “toner”) to form a toner image, transferring the toner image on the photoreceptor surface onto a surface of a recording material to such as paper via or not via an intermediate transfer body, and fixing this transferred image by heating, pressurizing, heating and pressurizing or a solvent steam. Toner remaining on the photoreceptor surface is cleaned by various methods if necessary, and is re-supplied to the aforementioned plural steps.
As a fixing technique for fixing a transferred image which has been transferred onto a surface of a recording material, a thermal roll fixing method of inserting a transfer material onto which a toner image has been transferred between a pair of rolls composed of a heating roll and a pressure roll to fix the image is common. In addition, as a like technique, a technique in which one or both of the rolls is substituted with a belt is also known. In these techniques, an image that is fixed fast can be obtained at high speed, energy efficiency is high, and damage to environment due to solvent volatilization or the like is minimal, as compared with other fixing methods.
On the other hand, in order to reduce the amount of energy to be used in a copying machine and a printer, a technique for fixing a toner using less energy is desired. For this reason, there is a great demand for a toner for electrophotography which can be fixed at a lower temperature.
As a means to lower the temperature for fixing a toner, a technique of lowering the glass transition point of a resin for a toner (binder resin) is usually implemented. However, when the glass transition point is too low, aggregation of a powder (blocking) is facilitated, and retainability of a toner on a surface of a fixed image is impaired and, therefore, practically, the lower limit is 50° C. This glass transition point is a design feature of resins for toner which are currently widely commercially available, and it has not been possible to obtain a toner which can be fixed at a yet further reduced temperature by means of further lowering the glass transition point. In addition, although the fixing temperature can be lowered by using a plasticizer, blocking occurs during storage of a toner or in a developing machine.
As a means for preventing blocking and realizing both image retainability up to 60° C. and low temperature fixability, a technique using a crystalline resin as a binder resin constituting a toner has been considered, and a method of using a crystalline resin as a toner for the purpose of realizing both blocking prevention and low temperature fixing has been long known (see, for example, JP-B No. 56-13943). In addition, for the purpose of offset prevention and pressure fixing, a technique of using a crystalline resin has been long known (see, for example, JP-B Nos. 62-39428 and 63-25335).
However, when in the above-disclosed techniques, for example, a polymer having an alkyl group side chain of a carbon number of 14 or more is applied to a toner, while the melting point thereof is low, at 62 to 66° C., this temperature is too low and, thus, there are problems of reliability of powder and images. When other crystalline resins are used, fixing performance on paper is not sufficient.
Examples of a crystalline resin for which improvement in fixability on a paper is expected include polyester resins. As a technique of using a crystalline polyester resin in a toner, technique of using a mixture of a non-crystalline polyester resin having a glass transition temperature of 40° C. or higher and a crystalline polyester resin having a melting point of 130 to 200° C. has been proposed (see, for example, JP-B No. 62-39428). In this technique, excellent fine grindability and blocking resistance are achieved. However, since the melting point of the crystalline polyester resin is high, fixing a yet further reduced temperature cannot be attained.
In order to solve the above problems, a technique using a crystalline resin having a melting point of 110° C. or lower, and using a toner with a non-crystalline resin mixed therein has been proposed (see, for example, JP-B No. 4-30014). However, when a non-crystalline resin is mixed in a crystalline resin, depression of the melting point of the toner occurs, thus, toner blocking occurs, and retainability of an image is deteriorated. When the non-crystalline resin component is large, since the properties of the non-crystalline resin component are greatly reflected, it is difficult to reduce the fixing temperature further than previously. For these reasons, unless a only crystalline resin is used as a resin for a toner, or a non-crystalline resin is mixed in an extremely small amount, practical use is difficult.
In view of the foregoing, it is desirable to use only crystalline polyester resin as far as possible in thermal roll fixing, and some examples using crystalline polyester resin have been proposed (see, for example, Japanese Patent Application Laid-open (JP-A) Nos. 4-120554, 4-239021, and 5-165252). However, in these techniques, the crystalline polyester resins are resins in which alkylene glycol or alicyclic alcohol having a small carbon number is used relative to a carboxylic acid component of terephthalic acid.
These polyester resins are described as crystalline polyester resins in the above references. However, since these polyester resins are essentially partial crystalline polyester resins, change in a viscosity relative to the temperature of a toner (resin) is not steep, and there are no problems in blocking property and image retainability. However, low temperature fixing cannot be realized in thermal roll fixing.
On the other hand, the present inventors have showed that a toner containing a crystalline polyester resin having a crosslinked structure as the main component is excellent in blocking resistance and image retainability, and can realize low temperature fixing (see, for example, JP-A No. 2001-117268). However, in such a toner, further improvement in charging property was desired in, particularly, two-component charging with a carrier.
Then, it was shown that a toner for electrophotography containing, as the main component, a crystalline polyester resin in which an ester concentration M of a crystalline polyester resin defined in the following equation (1) is not less than 0.01 and not more than 0.12, in the binder resin, can be obtained as a toner having further improved charging property (see, for example, JP-A No. 2002-82845).M=K/A  equation (1)(in the equation (1), M represents an ester concentration, K represents the number of ester groups in a polymer, and A represents the number of atoms constituting a polymer chain of a polymer)
On the other hand, in the specification of the above disclosure, a process for producing a toner for electrophotography is shown, comprising emulsifying a crystalline polyester resin containing, as a copolymerization component, 2 to 20 mole % of a di- or more valent carboxylic acid having a sulfonic acid group in which the ester concentration M is not less than 0.01 and not more than 0.12, and aggregating and fusing this to adjust to a toner diameter. In this case, it is described that preparation of a self-emulsifying solution of polyester is possible, particle size distribution of a toner obtained by the aggregation and incorporation is narrow, and transferring property thereof is better.
However, it was discovered that a toner prepared by an aggregating and incorporating method using this crystalline resin copolymerized with carboxylic acid having a sulfone group has a low charging property under high temperature and high humidity. While adjustment of the charging property can also be performed by using an external additive, improved charging property is preferable. Accordingly, it was desirable to find a new composition of a crystalline polyester resin in which charging property under high temperature and high humidity is maintained while low temperature fixing is realized, and an aggregation and incorporation method can be applied. Thus, a crystalline polyester resin has been proposed which is characterized in that two or more kinds of dicarboxylic acid-derived consitutional components are included as an acid-derived consitutional component constituting a polymer and the content of an aromatic dicarboxylic acid-derived consitutional component having no sulfonic acid group is in the range of 0.5 to 30 construction mole % of all acid-derived consitutional components. Further, when a dicarboxylic acid-derived consitutional component having a sulfonic acid group is included, the content of the dicarboxylic acid-derived consitutional component having a sulfonic acid group is 5 construction mole % or less of all acid-derived consitutional components. (See, for example, JP-A No. 2004-168827.)
However, a toner using this resin has a defect in that it is easily crushed and, when it remains on a photoreceptor, it cannot withstand a force received between a cleaning blade and an intermediate transference drum, is deformed, and remains on a photoreceptor or an intermediate transference drum. As a result, filming is encouraged on a photoreceptor or an intermediate transference body.